Photo-electric plant thinners

ABSTRACT

This application employs a plurality of photo-electric pneumatically actuated, plant-row-thinning assembly, there being a plurality of said assemblies in side-by-side relation on a transverse tool bar, each assembly haVing a pair of similar, axially spaced, integrally connected, pendulously suspended, relatively narrow, arcuate, photo-electrically actuated, rootcutting knives positioned upon opposite sides of a plant row which simultaneously and reciprocally swing laterally of the row so that when one knife exits from the row the other knife will simultaneously enter the row forwardly of the point of exit of the first knife to leave standing seedlings between the successive axial root-cutting paths of said knives. The knives of each thinning assembly are suspended from an axially extending knife shaft which is reciprocally rotated by means of an actuating lever extending upwardly from the knife shaft and terminating between a pair of horizontally aligned pneumatic pistons which are alternately pressurized to actuate the lever back and forth so as to reciprocate the pair of knives laterally of the row.

United States Patent Shader [4 1 Apr. 11, 1972 54 PHOTO-ELECTRIC PLANT THINNERS 3,135,083 6 /1964 Czajkowski 71/58 x [72] Inventor: Benjamin A. Shader, Denver, Colo. Primary Examiner Roben E Pulfrey [73] Assignee: The Eversman Mfg, Company, D Assistant Examiner-Stephen C. Pellegrino Colo. Attorney-R. H. Galbreath [21] Appl 12,288 This application employs a plurality of photo-electric pneu- L matically actuated, plant-row-thinning assembly, there being a Related Apphcamn Data plurality of said assemblies in side-by-side relation on a trans- [63] C ntinuati n-impart of Ser- No. 673,598, Oct. 9, verse tool bar, each assembly having apair of similar, axially 1967, Pat. No. 3,512,587. spaced, integrally connected, pendulously suspended, relatively narrow, arcuate, photo-electrically actuated, root- [52] 0.8. CI ..l72/6, 172/81, 171/58 cutting knives positioned p n pp sit sid s of a plant ro 51 Int. Cl ..A01b 63/00, AOlb 33/00 which simultaneously n ip y swing laterally of h 581 Field of Search ..172/6, 48, 517, 81; 171/57, row 50 that when one kmfe em m the ro h other kmfe will simultaneously enter the row-forwardly of the point of exit l7l/58 of the first knife to leave standing seedlings between the suc- 5 References Cited cessive axial root-cutting paths of said knives. The knives of each thinning assembly are suspended from an axially extend- UNITED STATES PATENTS ing knife shaft which is reciprocallfy rotated by fmean; of an actuating lever extending upwardly mm the kni e sha t and ter- 3,452,82l 7/1969 P6112 ..l72/6 minating between a p of horizontally aligned pneumatic 3533474 10/1970 "172/6 pistons which are alternately pressurized to actuate the lever 3512587 5/1970 Shader "172/6 X back and forth so as to reciprocate the pair of knives laterally 3,525,403 8/1970 Cayton et al ..l72/6 X Ofthe 3,308,890 3/1967 Rhode ..172/6 3,425,495 2/1969 Reeve et al. ..l72/6 X 9 Claims, 7 Drawing Figures i, Z 6 III 111- m2 J8 34 f4 57 z; 57 35 o 73 PATENTEDAPR 1 1 I972 3.654.998

SHEET 2 OF 5 IN VENTOR.

w v EFM/g MI/V 4 67/400? PATENTEDAPR H 1912 3. 654, 998

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PATENTEDAPR 1 1 I972 SHEET 5 OF 5 w 2 mm W m PHOTO-ELECTRIC PLANT THINNERS This application is a continuation-in-part of co-pending application Ser. No. 673,598filed Oct. 9, 1967 now US. Pat. No.

This invention relates to an automatically selective rowplant thinning assembly of the type employing an integral pair of relatively narrow axially spaced, root-cutting knives positioned upon opposite sides of, and travelling axially of, a plant row such as described in applicants co-pending application, Ser. No. 673,598, filed Oct. 9, 1967, now US. Pat. No. 3,512,587 of which this application is a continuation in part.

The purpose function, operation and use of the pair of knives is fully described in the said co-pending application. This application relates more specifically to a highly efficient pneumatic-motor assembly for imparting an efficient, pendulous, alternate, simultaneous reciprocation to the pairs of knives and to means for preventing undesirable, extraneous obstruction of the photo-electric light beam.

One of the principal difficulties encountered in this type of photo-electric plant thinners results from the physical destruction and operational inaccuracies resulting from mechanical vibration of the motor and knife assembly due to the relatively high speeds at which the knives are reciprocated. The principal object of this invention is to provide a highly efficient, pneumatic motor for actuation of the knives which will cushion the contacts and shocks to which such thinners are subjected and which will produce a rapid, quiet, accurate, smooth, cushioned, balanced reciprocation of the knives when m use.

Since thinning assemblies of this type must operate in all types of soils and under all types of field conditions, difficulties have been encountered in keeping the light path of a photo-electric beam continuously in an open unobstructed condition at all times so as to avoid undesirable light beam interferences. A further object of this invention is to provide highly efficient means for continuously pneumatically cleaning the light beam path and for efficiently elevating leaves, seedlings and other obstructions from the path of the passing photoelectric beam to avoid objectionable interference therewith. I

Other objects and advantages reside in the detail construction of the invention, which is designed for simplicity, economy and efficiency. These will become more apparent from the following description.

In the following detailed description of the invention, reference is made to the accompanying drawings which form a part hereof. Like numerals refer to like parts in all views of the drawings and throughout the description.

In the drawings:

FIG. 1 is a top plan view of one of the similar typical plantrow-thinning assemblies employed in this invention;

FIG. 2 is a partially broken away side elevational view of the disclosure of FIG. 1;

FIG. 3 is an enlarged rear forwardly directed partially broken away detail view of the improved Pneumatic-motorand-knife assembly, per se, of this invention looking in the direction of the arrow 3 in FIG. 1;

FIG. 4 is a similarly enlarged fragmentary, detail, axial-section of the pneumatic-motor-and-knife assembly taken on the line 4-4, FIG. 1;

FIG. 5 is a detail cross section of the motor-and-knife assembly taken on the line 5-5, FIG. 4;

FIG. 6 is an enlarged, fragmentary, detail, cross section taken on the line 6-6, FIG. 1; and

FIG. 7 is a similarly enlarged, fragmentary, broken away, detail cross section of the row thinning assembly of FIG. 6.

For multi-row operation, the invention employs an elongated, hollow, rectangular, completely sealed tool beam 12, which also serves as a compressed air reservoir and which is preferably provided with a suitable air compressor platform 61 as shown in the said co-pending application. The tool beam 12 is transversally supported in any conventional manner above the ground from a tractor, cultivator or other agricultural towing implement. A plurality of similar row-thinning assemblies such as shown in FIGS. 1 and 2 are clamped to the tool beam 12, by means of suitable conventional beam clamps 13, which are clamped to the beam by means of suitable clamp bolts 14, and which are spaced apart along the beam to individually align with the plant rows of the field being worked.

The row-thinning assemblies are similar and each employes an upper pair of similar forwardly extending parallel links 15 pivoted, as shown at 16, to the beam clamp 13 and a lower pair of similar forwardly extending parallel links 17 pivoted to the clamp 13, as shown at 18, vertically below the pivot 16. The links 15 and 17 are substantially similar in horizontal length and the rear extremities of the upper links 15 are pivoted, as shown at 19, between the upper extremities of a pair of vertical contoured frame bars 20. The lower links 17 are individually pivoted, as shown at 21, to the frame bars 20 vertically below the pivot 19 so that the frame bars 20 are maintained substantially vertical at all angular positions of the links 15 and 17.

The frame bars 20 are welded or otherwise secured to opposite sides of the rear extremity of a hollow, elongated,

rectangular, rearwardly extending, sealed, frame chest 22 which extends horizontally rearward from the frame bars 20, as shown in FIG. 2, and terminates in a fixed, vertical, transverse tail plate 23 to which the improved pneumatic motor and blade assembly, as shown in detail in FIG. 3, 4 and 5, is attached. The rear extremity of the frame chest 22 and the motor assembly are supported from the ground upon a pair of spaced-apart press wheels 24 which are adjustably and rotatably mounted in any conventional manner upon vertical axle brackets 25.

The pneumatic motor and blade assembly of this invention employes a hollow, vertical, substantially rectangular lever case 26 in which a knife shaft 27, having a hexagonal forwardly extending portion 28, is horizontally journalled in suitable bearings 29 positioned above and in alignment with the center line of the plant row.

The actual plant thinning is accomplished by two relatively narrow, relatively thin, arcuate knife blades, herein designated as the right front knife blade 30 and the left rear knife blade 31, having sharpended, row entering extremities 32 and 33, respectively. The knife blades 30 and 31 are formed on, or are respectively attached to, the lower extremities of oppositely positioned, arcuate knife arms 34 and 35 which are individually clamped upon the hexagonal portion 28 of the knife shaft 27 in axially adjustable spaced relation, as shown in FIG. 4, by means of suitable arm clamps 36. The knife arms 34 and 35 extend substantially radially downward from the hexagonal portion 28 of the knife shaft 27 and the knife blades 30 and 31 are arcuately concentric about the axis of the knife shaft. The knife arms are oppositely positioned and axially spaced so that the sharpened extremities 32'and 33 of the knife blades 29 and 30, though axially spaced apart, are substantially in axial alignment so that as each knife blade exits from the center line of the plant row the other blade will simultaneously enter the row from the opposite side as fully described in applicants said copending application Ser. No. 673,598.

The reciprocatory movements of the knife blades are attained by means of a shaft-actuating lever 37 which is secured, such as by means of set screws 38, to the knife shaft 27 between the bearings 29 and which extends upwardly to a rounded spring head 43 within the lever case 26, as shown in FIGS. 3 and 4. Reciprocatory bias is supplied to the lever 37 through the medium of a left cylinder 39 and a similar right cylinder 40 which are mounted inhorizontal alignment with the cylinder axes on opposite sides of the lever case 26, in any suitable manner such as by means of suitable clamp bolts 41 and cylinder heads 42.

The cylinders 39 and 40 are each provided with hollow pneumatic pistons 44 each of which is spring-pressed toward the lever 37 by means of a compression spring 45 extending axially from a spring cavity 46 formed in the adjacent cylinder head and a hollow spring cap 47, formed on the head of each piston 44, so thatthe springs 45 extend completely through their respective pistons and so that the spring caps 47 engage the opposite sides of the rounded spring head 43 of the lever 37.

A guide tube 48 extends between the spring caps 47 on the pistons 44 and frictionally surrounds the caps to maintain the pistons in axial alignment. The rounded spring head 43 of the actuating lever 37 extends freely through an opening 49 in the guide tube. Thus, the lever 37 and the blades 30 and 31 are resiliently cushioned and resiliently balanced between the springs 45 so that lost motion is substantially practically eliminated.

Vibration shocks are further reduced by forming a tubular plunger 96 on the outer face of each piston 44 which enters the spring cavities 46 in the adjacent cylinder heads 42, through an annular sealing gasket 98, when each piston approaches the extremity of its outward stroke to form a dash pot which cushions the retardation of the blades.

The successive positions of the blades are diagrammatically indicated in FIG. 3 by the dot-dash lines A," B and C. When there is pressure in the right cylinder 40 the dividing plane swings to the right as indicated by the line C so that the left blade 31 is undercutting as illustrated; when there is no pressure in the cylinders, the dividing plane aligns with the plant row as indicated at B so that neither blade is undercutting; and when there is pressure in the left cylinder 39 the dividing plane swings to the left as indicated at A so that the right blade 30 will be undercutting as described in said copending application.

Vibration damage is still further reduced by resiliently mounting the lever case 26 and its connected cylinders 39 and 40 from the frame chest 22. This is accomplished by forming a cup-like protuberance 50 on the front and back wall of the lever case 26, each of which is surrounded by a relatively thick cylindrical rubber cushion 51, and forming a circular projecting flange 52 on the lever case about each of the shaft bearings 29 each flange being surrounded by a thick circular rubber cushion ring 53. The rubber cushion 51 and the cushion ring 53 on the front wall are tightly fitted into receiving sockets 54 and 55, respectively, on the tail plate 23. The cushion 51 and the cushion ring 53 on the front wall of lever case are similarly tightly fitted into and are free to vibrate in similar receiving sockets 56 and 57 in a head plate 58 which is secured and clamped toward the tail plate 23 in any desired manner such as by means if clamp bolts 59. The head plate 58 is additionally supported by a lateral outwardly extending cross brace member 60 which is welded to the front of the head plate 58 and s secured to the frame of the assembly in any desired manner such as by clamp bolts 104, FIG. 1, such as by clamp bolts 104, FIG. 1.

Thus, it can be seen that the pneumatic and motor-blade assembly is free to universally vibrate to absorb vibrations between the latter assembly and the frame chest 22.

The actual actuation of the blades 30 and 31 is accomplished pneumatically. Compressed air is admitted, from a conventional, portable compressor (not shown) mounted on the compressor platform 61, to the row thinning units from the tool beam 12 and is fed through feed hoses 97 to a conventional four-way solenoid actuated air valve 62 mounted on the frame chest 22. Cylinder hoses 63 successively and alternately transmit and exhaust the air through the heads 42 of the cylin ders 39 and 40 to alternately actuate the pistons therein against the bias of the compression springs 45. The exhaust air from the cylinders is conducted from the four-way valve 62 through an exhaust conduit 64 to the frame chest 22 to maintain a reduced pressure therein for use in a photo-cell assembly.

The photo-cell assembly is mounted ahead of the blades as shown in FIG. 6 and comprises two vertical tubular legs 65 and 66 resiliently and adjustably mounted, as shown at 67, on horizontal light shield plates 68 secured beneath the frame chest 22 and the vertical frame bars 20. The legs are maintained in fixed I-I-shaped relation, by means of a cross yoke 69 welded thereto, and extend downwardly on opposite sides of a plant row. The legs are resiliently braced by means of brace pins 70 which extend outwardly into rubber sleeves 71 which are resiliently encased in sleeve sockets 72 on adjustable brackets 73 secured to side light shield plates 74 by means of adjustment bolts 75. The lower extremities of the legs 65 and 66 terminate in horizontal nozzle tubes 76 and 77, respectively, which are directed inwardly in alignment with each other. A photoelectric lamp 78 is positioned in the nozzle tube 76 and a photo-electric cell 79 is similarly positioned in the nozzle tube 77 by means of suitable bushings 80 from which the photo-electric circuits protrude as shown at 81 so as to direct a photo-electric beam 82 across the plant row as is conventional in photo-electric cell installations and as described in said co-pending application.

However, in this application, flexible exhaust conduits 83 are connected between a header 84 on the frame chest 22 and the upper extremities of the legs 65 and 66 which conduct streams of exhaust air from the chest to and through the nozzle tubes 76 and 77 to clear all debris from the path of the beam 82 when the device is in operation. Also in this application, the tube brackets 73 are adjustable so the height and axial position of the beam 82 may be adjusted to obtain the most efficient results in the rows being worked.

To still further clear the path of the photo-electric beam 82, a pair of cup-shaped, gauge wheels 85, provided with cupshaped rubber tires 86, are rotatably mounted on stub shafts 87 which are welded, as indicated at 88, in and project oppositely inward from the lower extremities of the vertical frame bars 20 on each side of the plant row as shown in FIG. 7. The concave sides of the cup-shaped wheels face inwardly toward each other and are provided with axial bearings 89 which rotate about inner bearing races 90 which are adjustably secured along the stub shafts 87 in any desired manner such as by means of set screws 91.

The inner extremities of the stub shafts 87 are similarly bevelled and pivot screws 92 are threaded into the inclined extremities to rotatably support circular leaf lifter discs 93 positioned in centered, V-shaped relation over the plant row as shown in FIG. 7. The discs 93 are resiliently flexible and are clamped between concentric clamping discs 94 mounted on hubs 95 rotatably mounted on the pivot screws 92. The V- shaped incline of the leaf lifter discs 93 is such as to place the lower peripheries of the two lifter discs in resilient contact with the seedlings along the plant row and the upper peripheries in flexed frictional contact with the concave surfaces of the gauge wheels as shown in FIG. 7; thus, the ground engagement of the wheels 85 drives the freely rotatable lifter discs in unison in a forward direction.

The spacing between the lifter discs and the spacing between the latter discs and the guage wheels can be pre-adjusted to accommodate field conditions by varying the position of the inner bearing races along the stub shafts 87. For the purposes of illustration, the lower peripheries of the two lifter discs have been shown in plant-engaging contact, as shown at D in FIG. 7, directly below the axial plane of the wheels. The position of the plant-engaging contact can, however, be pre-adjusted along the row by affixing the stub shafts 87 in pre-selective circumferential positions in the frame bars 20. Thus, if the two stub shafts are uniformly rotated slightly forward before being welded or otherwise affixed, the plantengaging contact D will be positioned rearwardly of the axial plane of the wheels. Therefore, the relative position along and above the row, between the photo-beam 82 and the plant-engaging contact, is fully pre-adjusted to obtain the most efficient results.

Thus, it can be seen that the gauge wheels will reduce the clods and produce a substantially flat topped row surface and the lower peripheries of the lifter discs will be substantially stationary as they engage the plants so that the leaves will be gently lifted and gathered above the photo-electric beam so that the plant stems can be clearly presented to the photoelectric beam.

The reciprocation of the knife shaft 27 is electrically controlled by a conventional micro-limit switch 99 which is mounted on an L-shaped switch plate 100 which is secured to the top of the lever case 26 by means of cap screws 101 and which extends forwardly and downwardly thereover. The micro-switch 99 is mechanically actuated by a radial contact actuating arm 102 mounted beneath a clamp nut 103 on the rear extremity of the knife shaft, as shown in FIGS. 3 and 7.

The electric circuits and electrical equipment involved are conventional for the purposes of this application. It will be apparent to those skilled in the art that any of a number of commercially available circuits may be employed for control of the thinner assemblies and the details thereof are not important to the understanding of the present application. By way of example, typical circuits are shown in U.S. Pat. Nos. to Harm 2,592,689, Chesebrough et al., 2,894,178, and Reeve et al., 3,425,495.

While a selected form of the invention has been above described, it is understood that mechanical variations and detail substitutions can be made by one skilled in the art without departing from the spirit of the invention.

Having thus described the invention what is claimed and desired to be secured by Letters Patent is:

1. In a plant thinning implement of the type having a knife shaft positioned over and in alignment with a row of plants and having a pair of axially spaced root-cutting knives depending therefrom so as to alternately enter said row, the combination with means for circumferentially reciprocating said knife shaft, said means comprising:

a. a lever case, in which said knife shaft is journalled, ex-

tending upwardly from the knife shaft;

b. a pneumatic cylinder extending outwardly from each side of said case in alignment with each other and above said knife shaft;

c. a shaft actuating lever secured to said shaft and extending upwardly into said lever case with its upper extremities terminating between said cylinders;

d. a compression spring in each cylinder said springs oppositely acting against the upper extremity of said actuating lever to resiliently maintain the latter vertical; and

e. pneumatic means in each cylinder acting to alternately and successively compress the spring in the other cylinder so as to force said lever back and forth against the bias of said springs.

2. A plant thinning implement as described in claim 1 in which the pneumatic means comprises:

a. a piston in each cylinder surrounding the spring therein and arranged to contact the upper extremity of said lever;

b. pneumatic pressure means in each cylinder acting to alternately urge said pistons toward said lever to actuate the latter back and forth against the bias of said springs; and

c. means for maintaining said pistons in axial alignment with each other and with the extremity of said lever.

3. A plant thinning implement as described in claim 2 in which the means for maintaining comprises:

a. an axially protruding spring cap on each piston in contact with said lever; and

b. a guide tube extending across the extremity of said lever and frictionally aligning the spring caps of said pistons together.

4. A plant thinning implement as described in claim 3 having:

a. means for allowing a limited relative movement between said lever case and the remainder of said plant thinning implement to provide a cushioning effect on the lever case.

5. A plant thinning implement as described in claim 4 in which the means for providing relative movement comprises:

a. protuberances formed on the front and rear external faces of said lever case; b. rubber cushion rings resiliently surrounding said protuberances; c. a tail plate and a head plate positioned, respectively, forwardly and rearwardly of said lever case;

d. cup-shaped sockets formed on said plates and resiliently and compressibly surrounding said cushion rings; and

e. means for securing said tail and head plates to said thinning implement.

6. A plant thinning implement as described in claim I having means for projecting a photo-electric beam across said row of plants comprising:

a. a pair of vertical tubular legs supported by said implement and positioned upon opposite sides of said row of plants forwardly of said knives;

b. a horizontal nozzle tube laterally mounted on and in communication with each of said legs, said tubes having open inner extremities facing each other;

e. a photo-electric lamp mounted in one of said tubes and a photo-electric cell mounted in the other of said tubes so as to project a light beam across said row between said open extremities; and

d. means for forcing air downwardly in said tubes to maintain the open extremities clear of obstruction.

7. A plant thinning implement as described in claim 6 hava. a pair of cup-shaped ground-driven gauge wheels rotatably positioned on opposite sides of the plant row forwardly of said tubular legs with their concave sides facing inwardly toward each other;

b. a flexible circular leaf lifting disc of smaller diameter than said gauge wheels rotatably positioned within the concave side of each gauge wheel, said discs being inclined so that their upper peripheries frictionally engage and are rotated by said gauge wheels and their lower peripheries approach each other adjacent said plant row.

8. A plant thinning implement as described in claim 6 hava. means acting to resiliently maintain said legs and said nozzle tubes in their desired positions.

9. In a plant thinning implement of the type having a reciprocably rotatable knife shaft positioned over and in alignment with a plant row said shaft having a pair of axially spaced root-cutting knives secured to and depending therefrom so that said knives will alternately enter the row, the combination with a. an elongated hollow pneumatic pressurized frame chest;

b. means for supporting said chest above and in alignment with a plant row;

0. means for mounting said knife shaft forwardly of and below said chest with the knives extending therebelow;

d. a pair of vertical tubular legs supported and extending downwardly from said frame chest forwardly of said knives and on opposite sides of said plant row;

e. a photo-electric lamp mounted in one of said legs and a photo-electric cell mounted in the other of said legs so as to project a light beam across said row;

f. a pair of similar cup-shaped ground-driven gauge wheels rotatably positioned on opposite sides of the plant row forwardly of said tubular legs with their concave sides facing inwardly toward each other; and

g. a circular flexible leaf lifting disc of smaller diameter than said gauge wheels rotatably positioned within the concave side of each gauge wheel, said discs being inclined with their upper peripheries in frictional engagement with their respective wheels and with their lower peripheries positioned closely adjacent to said plant row so as to be driven by said gauge wheels to lift the leaves of plants in advance of the approach of said light beam. 

1. In A plant thinning implement of the type having a knife shaft positioned over and in alignment with a row of plants and having a pair of axially spaced root-cutting knives depending therefrom so as to alternately enter said row, the combination with means for circumferentially reciprocating said knife shaft, said means comprising: a. a lever case, in which said knife shaft is journalled, extending upwardly from the knife shaft; b. a pneumatic cylinder extending outwardly from each side of said case in alignment with each other and above said knife shaft; c. a shaft actuating lever secured to said shaft and extending upwardly into said lever case with its upper extremities terminating between said cylinders; d. a compression spring in each cylinder said springs oppositely acting against the upper extremity of said actuating lever to resiliently maintain the latter vertical; and e. pneumatic means in each cylinder acting to alternately and successively compress the spring in the other cylinder so as to force said lever back and forth against the bias of said springs.
 2. A plant thinning implement as described in claim 1 in which the pneumatic means comprises: a. a piston in each cylinder surrounding the spring therein and arranged to contact the upper extremity of said lever; b. pneumatic pressure means in each cylinder acting to alternately urge said pistons toward said lever to actuate the latter back and forth against the bias of said springs; and c. means for maintaining said pistons in axial alignment with each other and with the extremity of said lever.
 3. A plant thinning implement as described in claim 2 in which the means for maintaining comprises: a. an axially protruding spring cap on each piston in contact with said lever; and b. a guide tube extending across the extremity of said lever and frictionally aligning the spring caps of said pistons together.
 4. A plant thinning implement as described in claim 3 having: a. means for allowing a limited relative movement between said lever case and the remainder of said plant thinning implement to provide a cushioning effect on the lever case.
 5. A plant thinning implement as described in claim 4 in which the means for providing relative movement comprises: a. protuberances formed on the front and rear external faces of said lever case; b. rubber cushion rings resiliently surrounding said protuberances; c. a tail plate and a head plate positioned, respectively, forwardly and rearwardly of said lever case; d. cup-shaped sockets formed on said plates and resiliently and compressibly surrounding said cushion rings; and e. means for securing said tail and head plates to said thinning implement.
 6. A plant thinning implement as described in claim 1 having means for projecting a photo-electric beam across said row of plants comprising: a. a pair of vertical tubular legs supported by said implement and positioned upon opposite sides of said row of plants forwardly of said knives; b. a horizontal nozzle tube laterally mounted on and in communication with each of said legs, said tubes having open inner extremities facing each other; c. a photo-electric lamp mounted in one of said tubes and a photo-electric cell mounted in the other of said tubes so as to project a light beam across said row between said open extremities; and d. means for forcing air downwardly in said tubes to maintain the open extremities clear of obstruction.
 7. A plant thinning implement as described in claim 6 having: a. a pair of cup-shaped ground-driven gauge wheels rotatably positioned on opposite sides of the plant row forwardly of said tubular legs with their concave sides facing inwardly toward each other; b. a flexible circular leaf lifting disc of smaller diameter than said gauge wheels rotatably positioned within the concave side of each gauge wheel, said discs being inclined so that their upper peripheries frictionally engage aNd are rotated by said gauge wheels and their lower peripheries approach each other adjacent said plant row.
 8. A plant thinning implement as described in claim 6 having: a. means acting to resiliently maintain said legs and said nozzle tubes in their desired positions.
 9. In a plant thinning implement of the type having a reciprocably rotatable knife shaft positioned over and in alignment with a plant row said shaft having a pair of axially spaced root-cutting knives secured to and depending therefrom so that said knives will alternately enter the row, the combination with a. an elongated hollow pneumatic pressurized frame chest; b. means for supporting said chest above and in alignment with a plant row; c. means for mounting saId knife shaft forwardly of and below said chest with the knives extending therebelow; d. a pair of vertical tubular legs supported and extending downwardly from said frame chest forwardly of said knives and on opposite sides of said plant row; e. a photo-electric lamp mounted in one of said legs and a photo-electric cell mounted in the other of said legs so as to project a light beam across said row; f. a pair of similar cup-shaped ground-driven gauge wheels rotatably positioned on opposite sides of the plant row forwardly of said tubular legs with their concave sides facing inwardly toward each other; and g. a circular flexible leaf lifting disc of smaller diameter than said gauge wheels rotatably positioned within the concave side of each gauge wheel, said discs being inclined with their upper peripheries in frictional engagement with their respective wheels and with their lower peripheries positioned closely adjacent to said plant row so as to be driven by said gauge wheels to lift the leaves of plants in advance of the approach of said light beam. 